All Thousand Oaks Optical H-Alpha filters are fine quality two-part systems. Each system consists of an Energy Rejection Filter (ERF) and an H-Alpha Filter Unit (HAU) with a fine-tuning mechanism. The HAU attaches either between the telescope back and the star diagonal or between the star diagonal and the eyepiece and accepts standard 1 1/4" accessories. The HAU is the same for all telescopes. The ERF fits over the front of the telescope tube and is instrument design specific. Telescopes with a secondary mirror obstruction (Schmidt Cassegrains) require an Off-Axis ERF while Refractors use a Center-Axis ERF. The purpose of the ERF is to increase the focal length and reduce the energy entering the telescope while allowing a narrow band of "red light" to enter. The HAU completes the blocking of all wavelengths except 656.28 nanometers, the wavelength emitted by the hydrogen atom. The HAU and ERF MUST BE USED TOGETHER.